Petrography, mineralogy and geochemistry of Balkan coals and their waste products

• The syngenetic pyrite cannot be completely separated out after coal flotation.
• It be supposed the presence of a dominant stagnant environment in the basin.
• Epigenetic framboidal pyrite was formed after a renewed microbial activity.
• Re-, REE-, W-bearing and other unusual phases were established.
• HREEs show an affinity to organic matter.

The samples studied include raw coal from five mines, coal concentrate, coal slurry, associated rock, and waste water. Collodetrinite comprises the dominant portion in raw coals and waste products while collotelinite and telinite/cryptotelenite dominate in the CC. The Ssulphide, particularly that of a syngenetic origin, cannot be completely separated out after coal crushing and flotation. The major minerals are pyrite, quartz, and kaolinite; the minor minerals — illite, muscovite, plagioclase, K feldspar, gypsum, calcite, dolomite, and siderite; and the additional mineral species as gold, sulphur, Fe1.24S0.76, pyrrhotite, galena, hematite, magnetite, hercynite, Cr-spinel, Ti-bearing minerals, biotite, halloysite, zircon, apatite, stilbite, celestine, Mn-calcite, Fe-dolomite, CaFe(CO3)2, ankerite biogenic minerals, volcanic ash, and possible cosmogenic dust are present in the samples as accessory phases. The modes of occurrence of the authigenic minerals suggest that this coal has undergone a series of syngenetic, epigenetic, and exogenic mineralisation. The origin of the epigenetic framboidal pyrite and neoform phases of microbial activity and water treatment is also discussed. The elements Те, S, Cd, U, Cs, V, Mo, W, and Cr are enriched in the coal concentrate compared with worldwide Clarke values of hard coal. Most of the elements have a mixed mode of occurrence. The Br, S, U, and Ge display a strong affinity to organic matter, whereas the Al, K, Si, Rb, Li, Ti, P, and Se display a distinctly inorganic pattern of distribution. Certain of the trace elements (e.g., Sr, Ba, P, Mn, Mo, As, Pb, Sb, Tl, Li, Nb, Be, Y, Ti, Yb, and Cd) are present as impurities in various minerals, whereas other trace elements (e.g., La, Ba, Cu, Re, Pb, Gd, Nd, Sr, Sn, and Cr) are present as discrete phases. It is suggested that the main sources of the trace elements were mineral and mixed sea waters, the basement rocks, and the Sliven U-polymetallic ore deposit. It was also revealed that a number of elements (S, Li, Cs, F, Br, NH−4, NO−3, and V) in the Balkan coals are mobile in water and may pose certain environmental concerns. The coal slurry could be used as a form of fuel.